This invention relates to a power supply circuit for supplying a load from input voltages of different magnitude, comprising a first series arrangement of a primary winding of a transformer and a main current path of a first semiconductor switch which also has a control input, and a second series arrangement of a secondary winding of the transformer and a rectifier diode. This second series arrangement has terminals for connection of the load. The power supply circuit includes a further semiconductor switching means provided with a control input for switching-off the first semiconductor switch, and first means for supplying the control input of the further semiconductor switching means with a first signal which is a measure of the current flowing through the primary winding.
Such a circuit can, for example, be used for recharging batteries or supplying electronic devices because it is possible to connect the apparatus, without switching them over, to voltage sources of different magnitude, for example, the electric power mains in different countries. Such a power supply circuit can more specifically be used in an electric shaver in which the circuit produces the current for supplying the motor or, in the case of a rechargeable shaver, for charging the batteries.
A power supply circuit of the type defined in the opening paragraph is disclosed in the European Patent Specification No. 0 030 026, which corresponds to U.S. Pat. No. 4,504,775 (3/12/85). Said Specification describes a power supply circuit in which, after the input voltage has been applied via a starter resistor, a small current flows into the base of a first transistor which acts as the first semiconductor switch, which causes this transistor to be partly rendered conductive. In response thereto, a small primary current flows in the primary winding of the transformer. As a results in the secondary winding, a voltage is generated which results in a larger current being applied to the base of this transistor via a positive feedback circuit coupled between the secondary winding and the base of the first transistor. The transistor is fully driven to the conductive state thereby. Thereafter the primary current increases linearly versus time, during what is commonly denoted as the forward phase. In the prior-art circuit the emitter line of the first transistor incorporates a resistor with which the base-emitter junction of a second transistor is arranged in parallel. This constitutes the further semiconductor switching means. In addition, the base of the second transistor is coupled via a resistor to the junction point of the secondary winding and the rectifier diode. Via this resistor a current flows from the secondary winding through the resistor incorporated in the emitter line of the first transistor. At a given value of the sum of this current and the primary current the second transistor becomes conductive and consequently the first transistor is rendered non-conductive. This causes the forward phase to be terminated and the start of what is commonly denoted the flyback phase. Because of the energy stored in the transformer a secondary current flows in the secondary winding during the flyback phase. This current constitutes the charging current for the batteries or direct supplies the current for the relevant apparatus. The secondary current decreases linearly with time until the first transistor is turned-on again.
Because of the coupling of the base of the second transistor to the secondary winding and the rectifier diode, the second transistor is rendered conductive at an instant which is earlier according as the input voltage is higher.
Thus, the power supply circuit forms a self-oscillating power supply which produces an average constant output current at a constant input voltage. However, in spite of said coupling, the output current in this power supply still depends to a significant extent on the input voltage.
From the European Patent Specification No. 01 88 839, which corresponds to U.S. Pat. No. 4,652,984, it is also known to provide a power supply circuit with a compensation of the output current which increases with increasing input voltages. In the power supply circuit illustrated in FIG. 6 of said European Patent Specifications this is achieved in that an RC-circuit, whose resistor is connected to the secondary winding of a transformer, is used to apply a signal to the further semiconductor switching means for switching these the further semiconductor switching means. At an increasing input voltage the level of the switching voltage across the capacitor is reached faster so that the further semiconductor switching means is switched-on.
Although the use of said measures considerably limits the increase in the output current for an increasing input voltage, there nevertheless remains in the prior-art methods of mains voltage compensation a clear dependence of the output current on the input voltage. Such a dependence is disadvantageous since, in order prevent damage to the load, the current through this load, for example, the charging current of a battery or the supply current of a motor, must not exceed a given maximum value. In the prior-art supply circuit the highest permissible output current is reached only at one single input voltage and is lower at input voltages deviating therefrom. However at these other input voltages the load does not function in an optimum manner. For example, charging a battery requires more time than is otherwise necessary or a connected motor does not supply its full power.